Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $56.84   13.172%   $64.327   $64.33   $64.35   $64.30  
  Purchase #2 $18.39   6.849%   $19.6495   $19.65   $19.65   $19.60  
  Purchase #3 $125.31   7.706%   $134.9664   $134.97   $134.95   $135.00  
  Purchase #4 $119.62   10.439%   $132.1071   $132.11   $132.10   $132.10  
  Purchase #5 $187.62   4.120%   $195.3499   $195.35   $195.35   $195.30  
  Purchase #6 $228.72   5.440%   $241.1624   $241.16   $241.15   $241.20  
  Purchase #7 $14.32   13.542%   $16.2592   $16.26   $16.25   $16.30  
  7 purchase totals = $750.82       $803.8215   $803.83   $803.80   $803.80